The present invention relates to a speed-changing device that switches the speed reduction ratio and is used in a power tool.
Japanese Laid-Open Patent Publication No. 2009-56590 describes a power tool including a reduction gear train that switches speed reduction ratios. The power tool automatically switches the speed reduction ratio of the reduction gear unit in accordance with the load torque applied to a motor.
The power tool detects the load torque from the current value of the motor. When the detected current value is greater than a predetermined value for a fixed time, the power tool switches the speed reduction ratio of the reduction gear train to lower the rotation speed of the motor. This prevents the temperature of the motor from suddenly increasing.
When the power tool is continuously used in a state in which the detected current value of the motor is slightly lower than the predetermined value, the speed reduction ratio of the reduction gear train is not switched. As a result, this may suddenly increase the motor temperature and heat the power tool thereby lowering workability.